Steel cords are commonly used for the reinforcement of elastomers such as rubber tires.
Particularly for dynamic applications such as rubber tires, a lot of requirements are Imposed on steel cords. Steel cords are required to have:
(a) a high breaking load;
(b) a high resistance against fatigue;
(c) a high resistance against corrosion.
(d) a low cord diameter
A high breaking load (a) can be obtained by twisting the steel cord from high-tensile strength filaments, i.e. steel filaments with a high-carbon steel composition which have been subjected to a high final deformation through a drawing operation so that a high tensile strength is obtained. Supposing an equal starting steel wire diameter for the half-product, the finer the final filament diameters the higher the deformation ratio and the higher the obtainable tensile strength. The high breaking load of the cord is then obtained by twisting the high-tensile filaments in a compact configuration, i.e. a configuration with much steel cross-section per total cross-section.
In order to obtain a high fatigue resistance (b) it is known to decrease the diameter of the individual steel filaments composing the steel cord and to increase the strength of the filaments and to avoid excessive internal wear (fretting) of the cord The decrease in diameter of the individual steel filaments also partially promotes the high breaking load (a) since the further drawing towards finer steel filaments increases the tensile strength.
A high resistance against corrosion (c) is obtained by open steel cord structures which allow elastomer or rubber to penetrate inside the steel cord between the individual steel filaments, so that moisture is prevented from traveling along hollow spaces inside and along the steel cord. This open character of the steel cord, however, is directly opposite to the goal of obtaining a high breaking load (a) and a high fatigue resistance, since it reduces the compactness and the amount of steel cross-section per total cross-section, and because for a high breaking load and high fatigue resistance a lot of small filaments are required, thus making it more difficult to achieve a fully open structure.
A low cord diameter (d) is required to reduce the amount of rubber used to embed the steel cords in, and hence the total cost and weight of the tire. However a low cord diameter is opposed to a high breaking load since less steel is available.